As is well known, a pyroelectric camera is basically a television camera that is capable of producing an infrared television picture. A pyroelectric vidicon tube is utilized as the input of the pyroelectric camera to sense a thermal image of a scene by receiving thermal energy emitted from the scene and, responsive thereto, an electrical output signal is provided, which electrical output signal is then processed by associated electronic circuitry to provide a composite video output signal.
A signal processing system for a chopper-type pyroelectric camera is shown and described in U.S. Pat. No. 4,481,535 issued Mar. 6, 1984 to Hodd and Bucher, and which patent is assigned to the assignee of this invention.
It is also known that operation of a pyroelectric camera requires that tube operation be stabilized, which occurs when the target is read by low energy electrons. Destabilization, however, occurs whenever the electrons arrive with sufficient energy to create a large number of secondary electrons then the target can accept from the reading beam in order to discharge. In this situation, the secondary electron emission coefficient is greater than one. The effect is that the target surface facing the electron gun of the camera tube is charged up to the field mesh potential (which is several hundred volts) and, if left in this condition for any prolonged length of time, the camera, or vidicon, tube is usually damaged. In addition, the tube is not capable of producing an image if in the crossover mode.
Destabilization of pyroelectric camera operation can occur during turn-on of the camera system, can also occur if parts of the target are not scanned, and/or can also be caused by a thermal image overload into the system that increases the pedestal level such that the secondary emission coefficient becomes greater than one.
The process to recover the target from crossover is well known and has been implemented by several different means in accordance with the recommendation from the manufacturers of pyroelectric tubes. The determination that the target is in crossover and the initiation for recovery, however, has heretofore had to be made by the operator since there has heretofore been no known reliable way to electronically detect this condition and automatically initiate recovery.